Speedometer gears
 

Does anyone know how many MPH each tooth on the speedo gear changes? I haven't pulled my gear out yet but my speedo is now 15 MPH slow so when I pull my gear out I need to know how to calculate which new gear I will need.
Thanks

Its not always 15mph slow. Having the wrong speedo gears will throw your speedo off by a percentage at all speeds. For example, if you have a 35 tooth driven gear but need a 37 tooth, your speedo will be off by 2/35 or 6% fast. More teeth on the gear will slow down the speedo drive, less will speed iit up.

This site worked for me to calibrate my speedo in my 87.

http://www.chevelles.com/shop/speedo.html

Here you go.

http://www.73-87.com/garage/speedo.htm

http://www.73-87.com/garage/spdgears.htm

I have no idea what size tires originlly came on the truck so I used the formula to find the right gear combo. Someone double check this before I go and buy the gears.

[18 t drive gear*3.42*(20168/35)]/1001=35.43~35 t driven gear


My tires actually measure 34.5 but I thought I'd put in 35 so I would run just a tad on the slow side since I tend to speed anyways.

35 or 36, either would be very close. 18 Teeth makes the teeth on the drive gear very small, but I don't think you have any other choice.

Don't like math?
Try website for the gears.
Just keep playing with the number of the drive gear until you find a driven gear that is closest to a whole number. Then just take the part numbers to a dealer or tell them the tooth numbers.